Catalysts for para-ethyltoluene dehydrogenation

ABSTRACT

Para-ethyltoluene dehydrogenation catalyst compositions and processes for using such catalysts are provided. The catalyst compositions comprise a catalytically active iron compound, e.g., iron oxide; a potassium catalyst promoter, e.g., potassium carbonate; an optional chromium compound stabilizer, e.g., chromic oxide, and a calcium compound, e.g., calcium oxide. Utilization of particular amounts of calcium compound in dehydrogenation catalyst compositions of this type will provide a catalyst especially suitable for promoting the selective dehydrogenation of para-ethyltoluene to form para-methylstyrene in improved yields.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved catalysts for the selective dehydrogenation of dialkyl aromatic hydrocarbons to produce alkyl vinyl aromatic hydrocarbons, more particularly to catalysts for the production of para-methylstyrene (PMS) via the dehydrogenation of para-ethyltoluene (PET).

2. The Prior Art

The vinyl benzenes play a particularly important role in the preparation of synthetic plastics and resins. The polymerization of styrenes, for example, to produce polystyrene resins is well known.

Styrene and styrene derivatives are typically produced from ethyl benzene materials by dehydrogenation over solid catalysts in the presence of steam, and at temperatures ranging from 500° to 700° C. The catalysts found to be the most effective for this process are those which are based on potassium oxide (carbonate) promoted, chromium oxide stabilized, iron oxide material. Considerable research has been directed toward attempts to improve the activity and selectivity of this class of catalysts. Any improvement which results in either increasing the selectivity (moles of desired product per mole of reactant reacted) or the conversion (moles of reactant reacted per mole of starting material) without lowering the other is economically attractive since the result is that the yield (moles of desired product produced per mole of reactant) of the product has been increased. Any increase in the numerical value of the yield results in a more efficient operation with more reactant being converted into the desired product. In commercial operations, many of which produce millions of pounds of product per year, a trade-off is frequently effected between selectivity and conversion. An increase of only 1 or 2 percentage points in the selectivity can result in a substantial savings of starting materials. An increase in conversion can substantially reduce capital expenditure and energy consumption. The trade-off may vary depending on raw materials costs, energy costs, and the age of the plant.

Attempts have been made to improve the conversion effectiveness and selectivity of iron oxide type dehydrogenation catalysts for use in various alkylaromatic dehydrogenation reactions. Riesser; U.S. Pat. No. 4,152,300; issued May 1, 1979, for example, discloses that an improvement in ethylbenzene dehydrogenation catalyst selectivity can be realized by incorporating small amounts of certain metal oxide materials into dehydrogenation catalyst compositions comprising mixtures of iron oxide, potassium oxide, vanadium oxide and, optionally, chromium oxide.

Courty; U.S. Pat. No. 4,134,858; issued Jan. 19, 1979, discloses an iron oxide based dehydrogenation catalyst containing particular amounts of clay to improve the conversion, selectivity and yield of styrene and divinylbenzenes produced by dehydrogenation of ethyl- or diethylbenzene. This U.S. Pat. No. 4,134,858 also notes that oxides of copper, vanadium, zinc, manganese, magnesium, nickel, cobalt, bismuth, tin and antimony can be added to the disclosed dehydrogenation catalysts.

Notwithstanding such attempts to improve iron oxide based dehydrogenation catalysts, there is a continuing need to formulate catalysts of this type which can be used to realize improved conversion, selectivity, and/or yield in the dehydrogenation of other types of alkylaromatic materials such as, for example, in the production of para-methylstyrene from para-ethyltoluene.

Accordingly, it is an object of the present invention to provide an improved iron oxide based dehydrogenation catalyst especially useful for the dehydrogenation of para-ethyltoluene to produce para-methylstyrene.

It is a further object of the present invention to provide a para-ethyltoluene dehydrogenation process employing a catalyst which provides a significant increase in selectivity to production of p-methylstyrene with little or no corresponding drop in para-ethyltoluene conversion.

These and other objectives can be achieved by means of the invention described and claimed herein.

SUMMARY OF THE INVENTION

The present invention relates to an improved dehydrogenation catalyst composition especially useful for the selective dehydrogenation of para-ethyltoluene to produce para-methylstyrene. Such a catalyst comprises from about 30% to 60% by weight of an iron oxide component, calculated as ferric oxide, from about 13% to 48% by weight of a potassium compound component, calculated as potassium oxide, from about 0% to about 5% by weight of a chromium compound component, calculated as chromic oxide, and from about 1% to 15% by weight of a calcium compound calculated as calcium oxide.

The present invention also relates to a dehydrogenation process wherein para-ethyltoluene, preferably along with steam, is passed over the calcium-containing catalyst composition at a temperature from about 500° C. to 700° C. with a LHSV of from about 0.3 to 3, to selectively produce para-methylstyrene.

DETAILED DESCRIPTION OF THE INVENTION

The dehydrogenation catalyst compositions of the present invention contain as an essential catalytic component one or more iron compounds, generally in the form of iron oxide. Many forms of iron oxide can be used in the catalyst compositions of this invention. Typically, iron oxides employed in catalyst preparations of this sort are a synthetically produced, powdered red, red-brown, yellow or black pigment. The red or red-brown pigments are highly pure ferric oxide, while the black pigment is the magnetic form, ferrosoferric oxide (Fe₃ O₄), which is usually found in the catalyst under various reaction conditions. The yellow iron oxides consist of the monohydrated form of ferric oxide. These oxides are prepared by various methods, e.g., oxidation of iron compounds, roasting, precipitation, calcination, etc. A suitable form of iron compound is the monohydrated yellow iron oxide used in the preparation of catalysts according to U.S. Pat. Nos. 3,360,597, issued Dec. 26, 1967, and 3,364,277; issued Jan. 16, 1968. Particularly suitable are pigment grade red iron oxides of purities exceeding 98% weight. These red oxides have surface areas ranging from 2 to 50 m.sup. 2 /gram and particle sizes from 0.1 to 2 microns. The iron compound is present in the catalyst in either one or a mixture of both of its possible oxidation states, i.e., as ferrous iron or ferric iron or mixtures thereof, as for example, ferrosoferric iron.

The catalyst compositions herein generally comprise from about 30% to 60% by weight, preferably from about 35% to 55% by weight, of iron oxide calculated as ferric oxide. Alternatively stated, the catalyst compositions herein generally comprise from about 21% to 42% by weight, and preferably from about 24% to 39% by weight, of iron oxide, calculated as iron metal.

The dehydrogenation catalyst compositions of the present invention also essentially comprise, as a catalyst promoter, one or more potassium compounds. The potassium promoter material can be added to the catalyst in various forms. For example, it may be added as the oxide, or as other compounds which are convertible, at least in part, under calcination conditions, to the oxides, such as the hydroxides, the carbonates, the bicarbonates, the phosphates, the borates, the acetates, and the like. A particularly preferred potassium compound is potassium carbonate. The potassium compound is generally present in the catalyst as a potassium oxide, a potassium carbonate or a mixture thereof. High carbon dioxide partial pressures in the reaction gases will favor high carbonate to oxide ratios and vice versa within the potassium component.

The catalyst compositions herein generally comprise from about 13% to 48% by weight, and preferably from about 27% to 41% by weight, of potassium promoter compound, calculated as potassium oxide. Alternatively stated, the catalyst compositions herein generally contain from about 11% to 40% by weight, and preferably from about 22% to 34% by weight, of potassium oxide, calculated as potassium metal.

An optional, but frequently utilized, third component of the present catalyst composition is a chromium compound which serves as a stabilizer for the active catalytic components. Chromium compounds have, in fact, typically been added to alkali-promoted iron oxide catalysts to extend their life. Chromium, as used in the compositions of this invention, can be added to the catalyst in the form of a chromium oxide or in the form of chromium compounds which decompose upon calcination to chromium oxides, as for example, chromium nitrates, hydroxides, acetates, and the like. Chromium can also be added in the form of alkali metal chromates. If potassium chromates are used, such materials can, of course, also contribute to the requisite concentration of potassium essentially present in the dehydrogenation catalyst compositions as hereinbefore discussed.

Thus, the catalyst compositions herein can comprise from about 0% to about 5% by weight, and preferably from about 1% to 4% by weight chromium compound, calculated as chromic oxide. Alternatively stated, the present composition can comprise from about 0% to 3.5% by weight, preferably from about 1.4% to 2.8% by weight, of a chromium oxide calculated as chromium metal.

In accordance with the present invention, the dehydrogenation catalyst compositions containing iron, potassium and optional chromium compounds, as described, also essentially contain particular selected amounts of a calcium compound which can provide calcium oxide in the catalyst compositions herein after calcination. Addition of calcium compounds to the particular iron-potassium chromium dehydrogenation catalysts utilized herein serves to enhance selectivity of production of p-methylstyrene when such catalyst compositions are used to promote the preferential formation of para-methylstyrene from para-ethyltoluene.

Calcium as used in the catalyst compositions of the present invention can be added to the catalyst in the form of calcium oxide, CaO, or in the form of other calcium compounds which decompose upon calcination to form calcium oxide, as for example, calcium carbonate and calcium hydroxide. Calcium oxide itself, i.e., lime, is the preferred calcium compound for use in formulating the compositions herein. Calcium compounds are added to the catalyst compositions of the present invention to the extent of from about 1% to 15% by weight, more preferably from about 2% to 10% by weight, calculated as CaO.

In addition to the foregoing materials, the catalyst compositions of the present invention can optionally contain a wide variety of materials suitable for altering, adjusting or modifying the catalytic and/or physical properties of such compositions. Materials, for example, which can act as stabilizers, activators, and promoters for dehydrogenation catalysts of the type herein contemplated include, cobalt, cadmium, aluminum, nickel, cesium, and rare earths. Such additives can be incorporated in various forms including their elemental form or in the form of their oxides. If employed, such stabilizers, activators and/or promoters generally comprise from about 1% to 15% by weight of the catalyst compositions herein.

It should be noted that the compositions of the present invention need not contain materials such as potassium aluminosilicate, e.g., kaliophyllite, in order to enhance catalyst activity and/or selectivity to production of p-methylstyrene products. The calcium oxide-containing catalysts of the present invention, in fact, can be maintained substantially free of clays or clay-like material without adversely affecting catalyst dehydrogenation activity or p-methylstyrene selectivity when used to promote dehydrogenation of p-ethyltoluene.

The physical strength of the catalyst compositions of the present invention can be improved, if desired, by adding any of a variety of optional insoluble binding agents. Binding agents can include, for example, calcium aluminate and Portland cement. It should be noted that such binding agents can contribute to the requisite calcium content of the catalyst compositions herein in the event that such calcium-containing binding agents form calcium oxide upon calcination of composites containing them. The density of the catalyst compositions herein can likewise be modified by the addition of various filler substances, for example, combustible materials such as sawdust, carbon, wood flour, etc. Such materials can be added to the compositions during preparation and thereafter burned out after the catalyst pellets have been formed. Other porosity promoting aids include graphite and aqueous solutions of methylcelluose, which also facilitate extrusion of catalyst pellets as hereinafter described. If employed, binders and other fillers generally can comprise up to about 20% by weight of the catalyst composition.

The catalyst compositions of the present invention are in general prepared by admixing the essential and desired optional components as hereinbefore described and by thereafter drying and optionally calcining the resulting mixture. Calcination temperatures can thus range from about 100° C. to 600° C., preferably from about 150° C. to 550° C. The compounds of the catalyst compositions herein can be admixed in various ways. One method comprises ballmilling together a mixture of the desired oxides and/or compounds decomposable upon calcination to oxides, adding a small amount of water, and extruding the paste formed to produce small pellets, which are then dried and calcined. Another method is to dissolve the components together, spray dry these components to form a resulting powder, calcine the powder into the resultant oxides, and then add sufficient water to form a paste which is extruded into pellets, dried and calcined. Another procedure involves precipitating those materials which are precipitatable, such as iron, chromium and calcium, as the resultant hydroxides, partially dewatering the resultant precipitate, adding soluble salts of the other desired metals, and then subsequently extruding, drying and calcining the resulting pellets. A preferred method involves dry-blend powdering of oxides and/or compounds decomposable upon calcination to the oxides, adding water, optionally containing dissolved therein soluble compounds decomposable upon calcination to the oxides, then mixing and/or mulling the resultant paste, pelletizing the mixture, subsequently substantially drying at a temperature from about 50° C. to about 300° C., followed by calcining the pellets to form the final product. The drying and calcining could be carried out stepwise in the same furnace by suitable programming of the furnace temperature. Alternatively, water-insoluble dry powders of oxides and/or compounds decomposable upon calcination to the oxides are dry-mixed, and the balance of the other materials needed are dissolved in water and the resultant solution is used to form the paste with the dry powders. There are many variations of the mixing of dry powders, water and water soluble compounds that give equivalent results and fall within the scope of this invention.

The catalysts of the present invention are especially effective in promoting the dehydrogenation of para-ethyltoluene to selectively produce para-methylstyrene. Such a dehydrogenation reaction is usually carried out at reaction temperatures of about 500° C.-700° C. However, higher or lower temperatures may be used without departing from the scope of this invention. The use of atmospheric, sub-atmospheric, or super-atmospheric pressure is suitable. However, it is preferable to operate at as low a pressure as is feasible, and atmospheric or sub-atmospheric pressure is preferred. The process of the invention may be carried out in batch, semi-continuous, or continuous operation, with continuous operation being preferred. The catalyst is employed in the form of a fixed bed, or in fluidized or suspended form. It is preferable to utilize a fixed bed. The reaction may be carried out in single stage reactors or by staging in series reactors. The reactors may be of various designs, e.g., downflow reactors, radial reactors, etc.

With the use of the catalyst of this invention, it is desirable to add steam to the reactant feed to aid in the removal of carbonaceous residues from the catalyst. The reaction feed generally contains from 2-30 moles of steam for every mole of organic feed. Catalysts having higher potassium contents are usually employed at lower steam to feed ratios. Steam to feed weight ratios of from about 1:1 to about 5:1 are desirable. Good results are obtained with steam to feed ratios of about 1.6:1 to about 4:1.

The contact time of the reactant-containing gas with the catalyst is usually defined in terms of liquid-hourly-space velocity (volume of liquid hydrocarbon reactant per volume of catalyst per hour, i.e., LHSV). The LHSV of the organic reactants according to this invention may vary from about 0.3 to 3 and is preferably adjusted within this range to effect the degree of conversion desired for the particular feed in question.

The catalysts of the present invention and their use will be further described by the following illustrative examples which are provided for illustration and are not to be construed as limiting the invention. It should be noted that advantages resulting from increases of selectivity and/or conversion of only one or two percentage points are extremely significant in a commercial process which may produce many hundreds of thousand pounds of product a day.

EXAMPLE I

A reference dehydrogenation catalyst is formulated by admixing the following materials in the following concentrations:

    ______________________________________                                         COMPONENT   CONCENTRATION (WT %)                                               ______________________________________                                         Fe.sub.2 O.sub.3                                                                           46%                                                                K.sub.2 CO.sub.3                                                                           51%                                                                Cr.sub.2 O.sub.3                                                                            3%                                                                            100.0%                                                             ______________________________________                                    

To this mixture is added water to the extent of 20% and the resulting mixture is formed into a paste. The paste is formed into 3/8" disks which are then dried for 16 hours at approximately 180° C. The disks are then crushed to 5-8 mesh particles.

EXAMPLE II

The dehydrogenation catalyst prepared as in Example I is used to dehydrogenate para-ethyltoluene (PET) at atmospheric conditions to form para-methylstyrene (PMS) in a tubular reactor maintained at 620° C. to which PET and steam are introduced. Feed rates are varied for two such dehydrogenation runs. Feed rates and the resulting conversion of p-ethyltoluene and selectivity to production of p-methylstyrene are set forth in Table I:

                  TABLE I                                                          ______________________________________                                         Dehydrogenation of Para-Ethyltoluene (PET) Over                                Reference Catalyst                                                             Feed Rates  Conversion Results                                                 Run  (LHSV)     PET Conversion                                                                              Para-Methylstyrene                                No.  PET    Water   (Mole %)   Selectivity (Mole %)                            ______________________________________                                         2    1.0    1.6     61.4       89.9                                            3    0.4    0.64    65.2       87.2                                            ______________________________________                                    

In carrying out such a reaction it was noted that, after several hours on stream, the exit port of the reactor and condenser used to recover the reaction products plugged up due to formation of "popcorn" polymer therein. To prevent such polymer formation, H₂ S was added at a rate ranging from 1 to 4 cc per minute of H₂ S (gas) per 100 ml per hour of PET (liquid) at the exit port of the tubular reactor.

EXAMPLE III

In a manner similar to that set forth in Example I, another dehydrogenation catalyst is prepared. The following dry materials are admixed in the concentrations shown.

    ______________________________________                                         Component    Concentration (wt. %)                                             ______________________________________                                         Fe.sub.2 O.sub.3                                                                            44.2                                                              K.sub.2 CO.sub.3                                                                            49.0                                                              Cr.sub.2 O.sub.3                                                                            2.9                                                               CaO          3.9                                                                            100.0%                                                            ______________________________________                                    

Water is added to the extent of 20% and the resulting paste is again formed into 3/8" disks and dried for 16 hours at 180° C. The dried disks are then crushed into 5-8 mesh particles. The resulting CaO-containing catalyst composition contains the same ratio of iron oxide to potassium carbonate to chromic oxide as does the reference catalyst of Example I.

EXAMPLE IV

Using the CaO-containing catalyst of Example III and the general reaction conditions of Example II, para-ethyltoluene (PET) is again dehydrogenated to para-methylstyrene (PMS). The conversion results are set forth in Table II:

                  TABLE II                                                         ______________________________________                                         Dehydrogenation of Para-Ethyltoluene (PET) Over                                Calcium Oxide-Containing Catalyst                                              Feed Rates  Conversion Results                                                 Run  (LHSV)     PET Conversion                                                                              Para-Methylstyrene                                No.  PET    Water   (Mole %)   Selectivity (Mole %)                            ______________________________________                                         1    1.0    1.6     60.5       91.4                                            2    0.4    0.65    64.7       90.1                                            ______________________________________                                    

A comparison of the conversion results from Tables I and II shows that the CaO-modified catalyst gives generally higher selectivity of the dehydrogenation reaction to formation of the desired p-methylstyrene in comparison with conversion over the unmodified catalyst. Such improved selectivity furthermore occurs without a significant offsetting loss of conversion of the p-ethyltoluene reactant.

EXAMPLE V

Two additional samples of iron-based dehydrogenation catalysts are made and again tested for their ability to promote dehydrogenation of para-ethyltoluene (PET) to form para-methylstyrene (PMS). A first sample containing only Fe₂ O₃, K₂ CO₃ and Cr₂ O₃ in the same proportions as those of the Example I catalyst is prepared and is tested for dehydrogenation performance against a similar catalyst sample containing 44.2 wt.% Fe₂ O₃, 49.0 wt.% K₂ CO₃, 2.9% Cr₂ O₃ and 3.88 wt.% CaO. Para-ethyltoluene dehydrogenation results using such catalysts, as well as the original base catalyst of Example I, are shown in Table III. Runs are made at a PET LHSV of 1 and at a temperature of 620° C. except where indicated.

                                      TABLE III                                    __________________________________________________________________________     Dehydrogenation of Para-Ethyltoluene-                                          Varying Reaction Conditions                                                    Run            Total Hours                                                                           % PET Conversion/% PMS Selectivity                       No.                                                                               Catalyst    on Stream                                                                             H.sub.2 O/PET = 2.0                                                                       H.sub.2 O/PET = 1.85                          __________________________________________________________________________     1  Example I-Unmodified                                                                       387    57-59/88-90                                                                               54-56/88-90                                   2  Example III-Unmodified                                                                     331    64-65/89-90                                                                               62-63/89-90                                   3  Example III-CaO                                                                            425    58-59/91-92(615° C.)                                                               59-61/91-92                                   __________________________________________________________________________

The Table III data indicate that addition of CaO to the catalysts of the type herein involved can improve the selectivity of such catalysts to production of para-methylstyrene during dehydrogenation of para-ethyltoluene. 

What is claimed is:
 1. In a dehydrogenation catalyst composition comprising the following components:(a) from about 35% to 55% by weight of iron oxide, calculated as ferric oxide; (b) from about 27% to 41% by weight of a potassium compound, calculated as potassium oxide; and (c) from about 0% to 5% of a chromium compound, calculated as chromic oxide,the improvement comprising combining with said components (a), (b) and (c) a modifying component (d), said modifying component (d) being capable of enhancing selectivity of production of p-methylstyrene when said catalyst composition is used to promote the preferential formation of para-methylstyrene from para-ethyltoluene, said modifying component (d) being a calcium compound, said calcium compound being present to the extent of from about 2% to 10% by weight of said catalyst composition, calculated as calcium oxide.
 2. A composition in accordance with claim 1 which is substantially free of clay materials.
 3. A composition in accordance with claim 1, wherein the chromium compound comprises from about 1% to 4% by weight of the composition.
 4. A composition in accordance with claim 3 which additionally contains up to about 20% by weight of a binder/filler component selected from the group consisting of Portland cement, calcium aluminate, sawdust, carbon, wood flour, graphite, methylcellulose and mixtures thereof. 